rc.subr: Support setting the audit user when starting services

When an unprivileged user restarts a service using, e.g., sudo, the
service runs with the audit user ID set to that of the unprivileged
user.  This can have surprising effects: for instance, a user that
restarts a jail that is running sshd will end up with their UID attached
to all audit logs associated with users who log in via that sshd
instance.  (sshd will set the audit user, but this is disallowed in
jails by default.)

Add support for rc.conf directives which cause rc to override the audit
user.  Specifically, make <name>_audit_user=foo cause the audit user to
be set to "foo" for service <name>.  A plain audit_user=foo directive
causes all services to be started as foo.

Note, like other similar rc features, this feature is limited to rc
services which are run by executing a command.  Shell functions can't be
wrapped this way.

Reviewed by:	0mp
MFC after:	2 weeks
Sponsored by:	Modirum MDPay
Sponsored by:	Klara, Inc.
Differential Revision:	https://reviews.freebsd.org/D53747

share/man/man5/rc.conf.5

@@ -22,7 +22,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd October 5, 2025
+.Dd November 14, 2025
 .Dt RC.CONF 5
 .Os
 .Sh NAME
@@ -190,6 +190,17 @@ Setting this option will bypass that check at boot time and
 always test whether or not the service is actually running.
 Enabling this option is likely to increase your boot time if
 services are enabled that utilize the force_depend check.
+.It Ao Ar name Ac Ns Va _audit_user
+.Pq Vt str
+A user name or UID to use as the
+.Xr audit 4
+user for the service.
+Run the chrooted service under this system group.
+By default, when an unprvileged user restarts a service using a utility
+such as sudo or doas, the service's will audit session will point to the
+unprivileged user, which may be undesirable.
+In that case, this variable can be used to override the audit user using
+.Xr setaudit 8 .
 .It Ao Ar name Ac Ns Va _chroot
 .Pq Vt str
 .Xr chroot 8
@@ -209,7 +220,6 @@ The
 value to run the service under.
 .It Ao Ar name Ac Ns Va _group
 .Pq Vt str
-Run the chrooted service under this system group.
 Unlike the
 .Ao Ar name Ac Ns Va _user
 setting, this setting has no effect if the service is not chrooted.